1. Field of the Invention
The present invention relates to a solenoid valve in which a spool valve is operated by the axial movement of a plunger corresponding to electric current applied to a coil.
2. Discussion of the Related Art
In solenoid valves of the type that a plunger is moved by a magnetic attraction force upon energization of a coil of a linear solenoid and that the areas of fluid paths opening to a valve hole are controlled by a spool valve which is moved within the valve hole as a result of the plunger being moved, variation is made volume of a rear end chamber to which the plunger enters its rear end portion upon movement thereof. It is described in Japanese unexamined, published patent application No. 2002-310322 to provide a respiration passage so that the operating oil can be flown into or away from the rear end chamber with the variation in the volume. That is, in the solenoid valve 1 described in the aforementioned Japanese application, a plunger 17 is slidably inserted into a stationary core 12 which has a receiving section 13, a magnetic resistance section 15 and an attraction section 14 in turn formed thereon, a coil 20 is contained in an annular space which is defined between the external surface of the stationary core 12 and the internal surface of a yoke 11, and a valve housing (sleeve) 41 of a spool control valve 40 is fixed to a forward end of the yoke 11. A spool 50 is slidably inserted into the valve housing 14 and is urged by a spring force toward abutting engagement with the plunger 17. A cover 30 is fixed to a rear end of the yoke 11 to cover a space 100 to which the rear end surface of the plunger 17 is exposed. The volume of the space 100 varies when the plunger 17 is moved by being magnetically attracted upon energization of the coil 20 of the linear solenoid 10. A respiration passage 101 is formed in the cover 30 for enabling the operating oil to flow into or out the space 100 in dependence on the variation in the volume of the space 100.
In the solenoid valve 1 described in the aforementioned Japanese application, by making the operating oil flow to and from the space 100, a damping effect on the movement of the plunger 17 by the action of the operating oil remaining in the space 100 is given in addition to the action of absorbing the volume variation due to the movement of the plunger 17. When the solenoid valve 10 is operated transitionally as is the case of ON/OFF operation of the coil 20, the plunger 17 and the spool 50 are moved sharply, and a surge pressure attendant on the sharp motion causes the plunger 17 and the spool 50 to generate the self oscillation. The self oscillation is suppressed by the damping effect of the operating oil reserved in the space 100 against the movement of the plunger 17.
Generally, the solenoid valve 1 described in the aforementioned Japanese application is used to be laid within an oil pan or the like storing the operating oil for an electronically controlled automatic transmission of a vehicle, with the axis of the plunger 17 extending horizontally.
However, in dependence on the driving state of the vehicle, it may be the case that the oil surface of the operating oil inside the oil pan comes down to be lower than the solenoid valve 1. In this case, the solenoid valve 1 is kept to operate for a long time in the atmosphere. In the solenoid valve 1 of the construction as described in the aforementioned Japanese application, the operating oil is discharged through the respiration passage 101 during the long time operation in the atmosphere, but no operating oil is supplied from outside to the space 100. Therefore, the damping effect of the operating oil against the movement of the plunger 17 cannot be achieved, whereby the solenoid valve 1 is liable to generate the self oscillation.